1. Field of the Invention
The present invention relates to an A/D converter and also to a method of testing the A/D converter.
2. Description of the Related Art
As is known, an A/D converter is a device which converts an analog signal to a digital signal. FIG. 1 shows a conventional A/D converter, which will be described below.
The A/D converter is incorporated within an integrated circuit (IC). It has two power-supply terminals 11 and 12. A high reference potential (e.g. 5 V) is externally applied to the first power-supply terminal 11, while a low reference potential (e.g., 0 V) is externally applied to the power-supply terminal 12.
The A/D converter further comprises resistors Ra and Rb and n resistors R1 to Rn, which are connected in series between the power-supply terminals 11 and 12. The resistor Ra is connected to one end of a series of the resistors R1 to Rn, and the resistor Rb to the other end of the series.
The resistors Ra and Rb determine the highest and lowest level of an input signal (i.e., an analog signal) which the A/D converter can detect. Neither the resistor Ra nor the resistor Rb will be required if the A/D converter is designed to detect signals at any level.
The resistors R1 to Rn determine the number of potential levels which are generated in the A/D converter. If the A/D converter is an 8-bit converter, requires 256 (=2.sup.8) resistors R1 to R256. The resistors R1 to Rn divide the voltage applied between the node A of the resistors Ra and R1 and the node B of the resistors Rb and Rn.
The potentials V1 to Vn-1 at the nodes of the resistors R1 to Rn are applied to two-input voltage comparators C1 to Cn-1, respectively. An input signal Vin is supplied to the voltage comparators C1 to Cn-1 from an input terminal 13. Each of the voltage comparators compares the potential of the input signal Vin with the potential at one node of resistors.
If the potential of the input signal Vin is higher than that at the node of resistors, each voltage comparator will output a high-level signal. Conversely, if the potential of the input signal Vin is lower than the potential at the node of resistors, the voltage comparator will output a low-level signal.
The signals P1 to Pn-1, thus output by the voltage comparators C1 to Cn-1 are supplied to an encoder 14. The encoder 14 converts the signals P1 to Pn-1 to a digital signal Dout which consists of m bits (e.g., 8 bits).
The input level which the A/D converter can detect and convert into a digital signal falls within a range which is determined by the potential Vtop at the node A and the potential Vbottom at the node B. This range will be broadest if neither the resistor Ra nor the resistor Rb is provided and if the highest potential (usually, the power-supply potential Vcc) applicable to the IC is applied to the power-supply terminal 11 and the ground potential is applied to the power-supply terminal 12. It follows that the difference between any adjacent two of the potential levels generated in the m-bit A/D converter is 2m (=n) times as low as the power-supply potential Vcc.
To determine the operating characteristic of an A/D converter which outputs a digital signal consisting of many bits, it is necessary to apply to the A/D converter various voltages which differ by a value equal to or less than the difference between any adjacent two of the potential levels generated in the A/D converter. Hitherto, an analog tester has been used to test multi-bit A/D converters. The analog tester can output various voltages which differ by such a small value, but is very expensive.
Furthermore, a signal input to the A/D converter may contain a noise which is greater than the difference between any adjacent two of the potential levels generated in the A/D converter. If this is the case, the analog tester can no longer serve to accomplish an accurate testing of the A/D converter.